This invention relates to a system for receiving sound pressure levels in a water environment and for modulating a carrier with said levels for transmission to a remote receiver, and more particularly to a sonobuoy which utilizes surface acoustic wave (SAW) resonators as a sound pressure level detector and as an information modulated reference frequency source for a selectable channel transmitter.
The reception of a target-emitted sound wave in a water environment is normally accomplished by means of a pressure sensitive device called a hydrophone comprising a quartz crystal. The device produces a voltage output proportional to the sound pressure level impinging on it and feeds the voltage to the transmitter unit where it is utilized to frequency modulate a carrier frequency. For example, in one of the prior art sonobuoys the gain compensated hydrophone output voltage causes a varactor diode to deviate the center frequency of a crystal oscillator. This technique is usable but causes degradation of the stability of the transmitter's carrier frequency. In such a system the target emissions are transformed from pressure to voltage and then from voltage to frequency before being placed in the proper format for transmission to a remote receiving station. Additionally in the band of interest, amplitude flatness extending toward DC can only be accomplished through the use of large components.
Another sonobuoy utilizes direct frequency modulation within a hydrophone by the impinging sound waves on it. In this case, the hydrophone comprises a surface acoustic wave (SAW) resonator operating at an RF frequency. These high operating frequencies make it impracticable to use the resonator at the normal hydrophone deployment depths of between sixty to a thousand feet. The signals from the hydrophone normally low in amplitude are excessively attenuated by the cable impedance which can only be minimized by the utilization of low loss coaxial cable. Such cables are heavy, expensive, occupy much space and are not practical for sonobuoy use. Addtionally the resonator used in this manner is inappropriate for multi-channel transmitter operation.